Fabrication of magnetoresistive random-access memory (MRAM) devices normally involves a sequence of processing steps during which many layers of metals and dielectrics are deposited and then patterned to form a magnetoresistive stack as well as electrodes for electrical connections. To define the magnetic tunnel junctions (MTJ) in each MRAM device, precise patterning steps including photolithography and reactive ion etching (RIE), ion beam etching (IBE) or their combination are usually involved. During RIE, high energy ions remove materials vertically in those areas not masked by photoresist, separating one MTJ cell from another. However, the high energy ions can also react with the non-removed materials, oxygen, moisture and other chemicals laterally, causing sidewall damage and lowering device performance.
To solve this issue, pure physical etching techniques such as pure Ar RIE or ion beam etching (IBE) have been applied to etch the MTJ stack. However, due to the non-volatile nature, physically etched conductive materials in the MTJ and bottom electrode can form a continuous path across the tunnel barrier, resulting in shorted devices. A promising approach to overcome this dilemma is to firstly partially etch the MTJ by physical RIE or IBE so that there is no chemical damage on the MTJ's free layer and the metal re-deposition is minimized. Secondly, either using additional photolithography or a self-aligned spacer hard mask, the remaining MTJ is etched by a chemical etch. In either case, the later etched MTJ portion has to be larger than the earlier etched portion so that the chemical damage on the MTJ's bottom portion does not affect the device performance, because its central effective portion, which is aligned with the free layer, is not chemically damaged. However, these additional complex photolithography or spacer deposition and etch steps result in a much higher fabrication cost. A simpler approach allowing for the same device structure and etch schemes is therefore needed.
Several patents teach two-step methods of etching MTJ stacks using spacers, including U.S. Pat. No. 9,087,981 (Hsu et al) and U.S. Pat. No. 9,406,876 (Pinarasi). Other patents disclose carbon hard masks, including U.S. Pat. No. 7,642,572 (Popp et al) and U.S. Patent Application 2017/0256708 (Krounbi et al), but these methods are different from the present disclosure.